The present work was carried out to study the properties
of both natural rubber composites reinforced by short polyaramid
fibres and natural rubber laminates reinforced by filament
polyaramid.
Initial work was concentrated to obtain a suitable
direct bonding agent and curing system for high modulus nylon
6,6 which was used as the control fibre. Two different types of
bonding and curing systems were established as useful for bonding
polyaramid to rubbers namely resorcinol-hexamethylene tetraminesilica
(HRH) and sulphur, and a blocked diisocyanate in conjunction
with a diurethane-crosslinker. Each system was found to offer a
different type of adhesion, the HRH formed physical bonds and the
NCO/urethane chemical links between fibre and rubber. When the
proportions of these bonding agents were optimised in the NR rubber
compound vulcanizate properties were significantly improved. Investigation established that for polyaramid fibres the
optimum aspect ratio of approximately 1250 was found to give
maximum composite reinforcement as judged by the tensile properties
of the composite. The experimentally found optimum of 4 parts of
blocked diisocyanate was used for extensive investigations of
composites properties with respect to fibre orientation and content.
Further studies on composites with polyaramid short fibres
demonstrated the necessity to optimize the proportions of blocked
diisocyanate in the composite to achieve maximum strength properties,
e. g. unoptimised with 30% fibre gave a tensile strength of 26.7 MPa
whereas optimised, with 30% fibre, gave a tensile strength of 34.4 MPa.
Post treatment of composites and laminates by heating or with
high energy radiation, using a Co60 source, improved the quality of
adhesion still further by the formation of more chemical links between
polyaramid and rubber though some degradation of rubber strength
simultaneously took place.
The bonding agent, diisocyanate, was found unsuitable for use in
a sulphur curing system, and was considered to form complexes with the
accelerator DCBS and sulphur, to react with zinc salts, and further
to act as a crosslinker for rubber. It was therefore found necessary
to vulcanize the rubber with a diurethane crosslinker which did not
show cure interference with the diisocyanate and the latter then
functions primarily as a bonding agent though a minor proportion is
shown to simultaneous crosslink the rubber. The Resorcinol-hexamethylene
tetramine-silica system. was found to contribute a type of physical
adhesion, possibly that of hydrogen bonding, between polyaramid and
rubber since this resin formed insitu during vulcanization of rubber was
found to crosslink the rubber; reaction mechanisms for both these
bonding systems are postulated.

Description:

A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.

Sponsor:

The present work was carried out to study the properties
of both natural rubber composites reinforced by short polyaramid
fibres and natural rubber laminates reinforced by filament
polyaramid.
Initial work was concentrated to obtain a suitable
direct bonding agent and curing system for high modulus nylon
6,6 which was used as the control fibre. Two different types of
bonding and curing systems were established as useful for bonding
polyaramid to rubbers namely resorcinol-hexamethylene tetraminesilica
(HRH) and sulphur, and a blocked diisocyanate in conjunction
with a diurethane-crosslinker. Each system was found to offer a
different type of adhesion, the HRH formed physical bonds and the
NCO/urethane chemical links between fibre and rubber. When the
proportions of these bonding agents were optimised in the NR rubber
compound vulcanizate properties were significantly improved. Investigation established that for polyaramid fibres the
optimum aspect ratio of approximately 1250 was found to give
maximum composite reinforcement as judged by the tensile properties
of the composite. The experimentally found optimum of 4 parts of
blocked diisocyanate was used for extensive investigations of
composites properties with respect to fibre orientation and content.
Further studies on composites with polyaramid short fibres
demonstrated the necessity to optimize the proportions of blocked
diisocyanate in the composite to achieve maximum strength properties,
e. g. unoptimised with 30% fibre gave a tensile strength of 26.7 MPa
whereas optimised, with 30% fibre, gave a tensile strength of 34.4 MPa.
Post treatment of composites and laminates by heating or with
high energy radiation, using a Co60 source, improved the quality of
adhesion still further by the formation of more chemical links between
polyaramid and rubber though some degradation of rubber strength
simultaneously took place.
The bonding agent, diisocyanate, was found unsuitable for use in
a sulphur curing system, and was considered to form complexes with the
accelerator DCBS and sulphur, to react with zinc salts, and further
to act as a crosslinker for rubber. It was therefore found necessary
to vulcanize the rubber with a diurethane crosslinker which did not
show cure interference with the diisocyanate and the latter then
functions primarily as a bonding agent though a minor proportion is
shown to simultaneous crosslink the rubber. The Resorcinol-hexamethylene
tetramine-silica system. was found to contribute a type of physical
adhesion, possibly that of hydrogen bonding, between polyaramid and
rubber since this resin formed insitu during vulcanization of rubber was
found to crosslink the rubber; reaction mechanisms for both these
bonding systems are postulated.